Schottky diodes have been used as infrared (IR) sensors to form focal plane arrays which can be integrated with CCD (charge coupled device) addressing circuit and other image process circuits. The Schottky diodes have remarkable diode to diode photoresponse uniformity, wide dynamic range, and wide operating temperature range. However, the quantum efficiency of Schottky IR sensors is low due to the inherent internal emission detection mechanism, which limits operation to low f-number systems.
One possible approach to increase the quantum efficiency is using vertically stacked Schottky diodes. This apparently can only be achieved by alternative growth of epitaxial silicon and silicide layers by molecular beam epitaxy (MBE).
Use of closely packed arrays of p-n or Schottky junctions oriented perpendicular to the plane of the silicon wafer has been proposed and demonstrated for the efficient detection of visible and near-IR light. However, to achieve a high quantum efficiency for IR light, the lateral dimensions of the metal layers must be reduced so that the photo-excited electrons will be collected by the Schottky junction before losing their energy through collision with cold electrons. Also, to achieve the effect of stacked Schottky diodes, the lateral dimensions of metal layers should be less than the IR absorption length of the metal layers, which is about 200 .ANG.. Thus, trenches must be patterned with lateral dimensions of about 200 .ANG. and depth of several micrometers and later filled with metal to form such Schottky devices. This is difficult at best, if not impossible, with present lithographic and etching technologies.